The default format for DATE to character conversion uses the format in effect for the DATE value.
To override the default format, you can convert a DATE value to a string using a FORMAT phrase. The resulting format, however, is the same as the DATE value. If you want a different format for the string value, you need to also use CAST as described here.
You must use nested CAST operations to convert values from DATE to CHAR and force an explicit FORMAT on the result regardless of the format associated with the DATE value. This is because of the rules for matching FORMAT phrases to data types.